GB1309449A - Gas assisted electrostatic pinning - Google Patents

Abstract

1309449 Electrostatic pinning of films E I DU PONT DE NEMOURS & CO 11 May 1970 [12 May 1969 23 March 1970] 22677/70 Heading B5B [Also in Division H1] Apparatus for electrostatic pinning of a thermoplastic dielectric web or film to a chilling drum in manufacture (Fig. 1) comprises a hopper lip 30 from which film is extruded at 33 to contact the periphery of a moving quench drum 29; an electrode 25 transverse of the web being connected to direct high voltage source 15 and a gas supply pipe of dielectric material being provided with a transverse slot 32 to maintain a gaseous atmosphere round the wire; whereby the film is charged electro-statically to adhere to the drum. A point probe for electrostatic charging may comprise (Fig. 2, not shown) an elongated electrode surrounded by a dielectric sleeve to which gas is supplied for ejection round the electrode point. In a modification (Fig. 4) wire 25 is embraced by a concentric sector of a cylinder comprising an inner dielectric cylinder and an outer earthed electrode 41 through which a stream of gas is passed from a source 16. Adhesion force may be tested (Fig. 3, not shown) by drawing film from a roll across a polished earthed stainless steel block by a spring balance registering adhesive force; the film being electrostatically charged in a gas atmosphere by a point probe (Fig. 2, not shown) supplied from a gas source over a flowmeter with electrode voltage the maximum permissible without breakdown. The gas is, e.g. nitrogen, helium, dry air, oxygen, dichlorotetra fluoro ethane, dichloro difluoromethane, tetrochloro ethane in nitrogen, acetone vapour in nitrogen, carbon tetrachloride in dry air or nitrogen; and is tested by passage through a transparent dielectric chamber 50 (Fig. 5) containing an insulated earthed steel plate 53 masked by dielectric film 57 leaving a central gap over which a tensioned wire electrode 58 is positioned spacedly at right angles; the spacing determined by blades 59. After purging with dry air, test gas is admitted and direct voltage is increasedly applied to the wire over a microammeter, and the average voltages for threshold current and for spark breakdown are determined; after which the microammeter current at 0À1 kv. before breakdown is determined as "wire current before breakdown". The desirable gas has the last factor at least 100 ÁA per inch of exposed wire, and margin of at least 2 kv. between threshold current voltage and spark breakdown voltage. A melt-cast of film polyethylene terephthalate is exemplified.